Netflix I "think / read" is only using FreeBSD with their Storage Appliance. So that is relatively small parts in number of Servers. Most of their operation, those Chaos Monkey killed machines are all EC2 and Linux.
Whatsapp is Erlang and FreeBSD, a real rare bleed. But i read Facebook is hiring Engineers to make Linux Network Stack better then FreeBSD. Bold claim on the Job Description, then there were rumours Whatsapp moving to integrate with Facebook, i.e moving off FreeBSD to Linux. I doubt thats an coincidence.
Despite the many / some similarities between FreeBSD and OSX, even Apple aren't using any FreeBSD on their Servers. At least there hasn't been any evidence in hiring.
So really, which large, Internet Company is actually using FreeBSD?